The present invention relates to semiconductor processing equipment, and more particularly to carriers for holding a semiconductor wafer during chemical-mechanical planarization.
Semiconductor wafers are planarized or polished to achieve a smooth, flat finish before performing process steps that create electrical circuits on the wafer. This polishing is accomplished by securing the wafer to a carrier, rotating the carrier and placing a rotating polishing pad in contact with the rotating wafer. The art is replete with various types of wafer carriers for use during this polishing operation. A common type of carrier is securely attached to a shaft which is rotated by a motor. A wet polishing slurry, usually comprising a polishing abrasive suspended in a liquid, is applied to the polishing pad. A downward polishing pressure is applied between the rotating wafer and the rotating polishing pad during the polishing operation. This system required that the wafer carrier and polishing pad be aligned perfectly parallel in order to properly polish the semiconductor wafer surface.
The wafer carrier typically was a hard, flat plate which did not conform to the surface of the wafer which is opposite to the surface being polished. As a consequence, the carrier plate was not capable of applying a uniform polish pressure across the entire area of the wafer, especially at the edge of the wafer. In an attempt to overcome this problem, the hard carrier plate often was covered by a softer carrier film. The purpose of the film was to transmit uniform pressure to the back surface of the wafer to aid in uniform polishing. In addition to compensating for surface irregularities between the carrier plate and the back wafer surface, the film also was supposed to smooth over minor contaminants on the wafer surface. Such contaminants could produce high pressure areas in the absence of such a carrier film. Unfortunately, the films were only partially effective with limited flexibility and tended to take a xe2x80x9csetxe2x80x9d after repeated usage. In particular, the set appeared to be worse at the edges of the semiconductor wafer.
The wafer carrier described in U.S. Pat. No. 5,762,544 typifies another problem associated with many prior wafer carrier designs. U.S. Pat. No. 5,762,544 discloses use of a flat, rigid carrier base that was connected to a shaft through a gimballing mechanism intended to keep the carrier base surface parallel to the semiconductor wafer surface during polishing. Typically, the arrangement resulted in applying one pressure across the entire semiconductor wafer surface. Thus, changing the force transferred through the shaft to the carrier base resulted in altering the applied pressure across the entire surface of the semiconductor wafer. The problem with using wafer carriers like the one described in U.S. Pat. No. 5,762,544 is that despite the apparent application of uniform pressure over the wafer surface, some planarization methods form one or more annular depressions near the perimeter of the wafer on the surface upon which circuit deposition is to occur. Only sufficiently smooth, flat portions of the wafer surface can be effectively used for circuit deposition. Thus, the annular depressions limit the useful area of the semiconductor wafer.
Other wafer carrier designs, such as described in U.S. Pat. No. 5,762,539, implement means for applying more than one pressure region across the back surface of the semiconductor wafer to attempt to compensate for uneven removal patterns, such as the annular depressions noted above. Specifically, the carrier described in U.S. Pat. No. 5,762,539 provides a top plate with a plurality of internal chambers that may be independently pressurized. A plurality of holes penetrate the top plate and a pad abutting the bottom surface of the top plate. By pressurizing the individual chambers in the top plate to different magnitudes, different pressure distributions can be established across the wafer surface abutting the pad; however, the pressure distributions are not sufficiently controllable to establish distinct areas across the back surface of the wafer having the same applied pressure. This is because pressurized fluid is directly applied to the back surface of the wafer through the tiny holes in the top plate, and the pressurized fluid is substantially free to move across the wafer""s back surface. Thus, pressurized fluid applied to one area of the back surface of the wafer moves into adjacent areas of the wafer""s back surface being supplied with a pressurized fluid at a different pressure. Therefore, the ability to control the applied pressure across specified, distinct sections of the wafer is limited, thereby restricting the ability of the design to compensate for anticipated removal problems.
There therefore was a need to provide a carrier design permitting controlled application of multiple pressure regions across the back surface of a semiconductor wafer during polishing.
A general object of the present invention is to provide an improved wafer carrier for polishing semiconductor wafers.
Another object is to provide a wafer carrier which applies uniform pressure over the entire area of the semiconductor wafer, if desired.
Yet another object of the present invention is to provide a wafer carrier which applies non-uniform, yet controlled pressure over the entire area of the semiconductor wafer to compensate for anticipated, troublesome removal patterns such as a perimeter annular depression or a centrally located bulge typically referred to as a center slow problem.
A further object of the present invention is to provide a surface on the carrier which contacts the back surface of the semiconductor wafer and conforms to any irregularities of that back surface. Preferably, the surface of the carrier should conform to even minute irregularities in the back surface of the semiconductor wafer.
These and other objectives are satisfied by a carrier for an apparatus which performs chemical-mechanical planarization of a surface of a workpiece that includes a rigid plate having a major surface. The carrier also includes a first diaphragm of soft, flexible material with a first section for contacting a first surface portion of the workpiece. The first diaphragm is connected to the rigid plate and extends across at least a first portion of the major surface, thereby defining a first cavity therebetween.
The carrier also includes a second diaphragm of soft, flexible material with a second section for contacting a second surface portion of the workpiece. The second diaphragm is also connected to the rigid plate and extends across at least a second portion of the major surface, thereby defining a second cavity therebetween. A plurality of fluid conduits provides pressurized fluid, such as a gas, that is connected to one or more of the cavities.
By pressurizing the cavities to the same or to different pressures, as desired, one can apply a uniform or a controlled, non-uniform pressure distribution over the workpiece surface, respectively. Additionally, since the diaphragms are made from a soft, flexible material, such as polyurethane, or nitrile rubber, or butyl rubber, the diaphragms, which contact the back surface of the workpiece, conform to any irregularities of that back surface.
In the preferred apparatus embodiment of the present invention, only two diaphragms having associated cavities and an inter-diaphragm cavity are included; however, in general, any desired number of diaphragms with their respective cavities and inter-diaphragm cavities may be implemented. Additionally, regardless of the selected number of diaphragms, they may be separate diaphragms connected together or one integral diaphragm having the desired number of independent cavities.
In another embodiment of the present invention, the carrier comprises a rigid plate having a major surface with a plurality of cavities formed therein, a diaphragm of flexible material coupled to and abutting a portion of the major surface, a first member coupled to and abutting a lower surface of the diaphragm, a second member coupled to and abutting the lower surface of the diaphragm, and a plurality of fluid conduits by which a source of pressurized fluid, such as a gas, is connected to at least one of the cavities. As in the prior embodiment of the carrier, appropriate pressurization of the carrier cavities in this later embodiment can compensate for otherwise uneven removal rates during polishing of the workpiece.
The present invention also provides a method for controlling the chemical-mechanical planarization of a surface of a workpiece to compensate for uneven removal rates on the surface comprising: providing a rigid plate having a major surface; pressurizing a first cavity formed by a first diaphragm of soft, flexible material and by a first portion of the major surface of the rigid plate to permit a first section of the first diaphragm to contact a first surface portion of the workpiece which is located on a side that is opposite the surface of the workpiece; pressurizing a second cavity formed by a second diaphragm of soft, flexible material and by a second portion of the major surface of the rigid plate to permit a second section of the second diaphragm to contact a second surface portion of the workpiece which is located on a side that is opposite the surface of the workpiece; selecting pressurization of the cavities to compensate for the uneven removal rates; and polishing the surface of the workpiece.
During polishing, the cavities are pressurized with fluid, such as a gas, which causes the diaphragms to exert force against the workpiece pushing the workpiece into an adjacent polishing pad. Because the diaphragms are made from a thin, soft, and highly flexible material, the diaphragms conform to the back surface of the workpiece which is opposite to the surface to be polished. By conforming to even minute variations in the workpiece surface, the diaphragms exert pressure evenly over the entire back surface of the workpiece, thereby producing uniform polishing.
These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefor, to the claims herein for interpreting the scope of the invention.